Fiber lasers are becoming the laser of choice in numerous industrial applications. More and more, these applications require the laser to emit at the kilowatt power level which puts a strain on the power handling capabilities of laser components. A key element of this revolution, is the development of double clad optical fibers (DCOF), presented in FIG. 1. These fibers serve as brightness converters between low brightness/low beam quality multimode pump diodes and the high brightness/high beam quality signal coming out of the signal guide 1.
One of the most common causes of failure in fiber lasers is the thermal degradation of the pump guide's 2, low index polymer pump guide cladding 3 and of the high index acrylate protective jacket 4 at the junctions of the fibers. At these junctions, the optical perturbation from splicing the fibers transfers some of the optical power into modes that are no longer guided by the fiber waveguide structure. While excess loss of the splice may be guided by the glass-air interface of the uncoated fiber after the splice, any light traveling at a numerical aperture superior to the numerical aperture of the low index polymer is immediately stripped as it reaches the coated fiber interface and absorbed in the low index polymer 3 and acrylate protective jacket 4. In turn, the resulting excessive heating may cause thermal degradation to the fiber's low index polymer 3 and to the acrylate protective jacket 4, rendering them even more absorptive to light, ultimately causing the splice to burn or melt and triggering a catastrophic failure of the fiber laser. U.S. Pat. No. 6,515,994 by Di Giovanni et al. describes a technique for detecting thermal power emanating from a fiber splice as a result from absorption of the light carried by the fiber.
Another common mode of failure in fiber lasers occurs in the first few centimeters of the amplifying fiber after the entry point of the pump power. In the doped fiber core, quantum defect result in pump radiation being converted into heat in the glass host matrix. For example, the lasing of ytterbium ions is accompanied by about 20% conversion of the pump power into heat. The associated temperature rise of the fiber core in turn leads to a temperature increase of the low index polymer 3 and the fiber's acrylate protective jacket 4. Lifetime studies on acrylate fiber coatings have suggested maintaining the coating temperature below about 80° C. Consequently, commercial high power fiber lasers often have to be designed to avoid such problems by distributing the pump power at a number of entry points, thus preventing that the coating temperature exceeds its operating temperature.
FIG. 2 depicts exemplary optical fiber components of an optical fiber device and indicates possible sites of catastrophic failures due to thermal degradation.
U.S. Patent Application Publication No. 2007/0206909 by Wetter et al. describes an optical fiber component package for high power dissipation. FIG. 3 shows such a package 10′ for an optical fiber device 12′ according to Wetter. The optical fiber component package has a high thermal conductivity packaging substrate, 18′ and 20′, surrounding the optical fiber device 12′ and has adhesive bonds, 14′ and 16′, at each end anchoring the optical fiber device to the substrate. The adhesive bonds are made of a material that has high transparency in damp heat as well as a high glass transition temperature and is capable of withstanding temperatures of over 100° C. However, the proposed adhesives in Wetter et al. are poor thermal conductors, which can cause thermal degradation and failure. The fact that adhesive bonds are a common cause of thermal failure has been identified in F. Séguin at al., Tapered fused bundle coupler package for reliable high optical power dissipation, Fiber Lasers III: Technology, Systems, and Applications, Proceedings of SPIE Vol. 6102, 2006. Ideally, the adhesive bond should be a good thermal conductor and be optically transparent. However, to the knowledge of the inventor, adhesive material that is simultaneously a good thermal conductor and optically transparent is not currently available.
In view of the above, there remains a need for a simplified device for preventing thermal degradation of optical fiber components under high power operation.